In a manufacturing process of semiconductor devices, a plasma processing apparatus for performing an etching process or a film forming process on a semiconductor wafer (hereinafter, referred to as a “wafer”) by high density plasma generated at a relatively low pressure atmosphere conventionally has been used. For example, in case of a parallel plate type plasma processing apparatus, a pair of parallel plate type electrodes, i.e., an upper electrode and a lower electrode, are disposed in a processing chamber, a processing gas is introduced into the processing chamber and at the same time radio frequency power is supplied to either or both electrodes from a radio frequency generator to generate a radio frequency electric field between the electrodes. The radio frequency electric field forms plasma of the processing gas to thereby perform specified processes such as an etching process and the like on the wafer.
To achieve a desired process result by the plasma processing apparatus described above, it is important to maintain the radio frequency power applied to the electrodes at a specified level during the processing so that the plasma generated in the processing chamber can be stabilized. However, if several plasma processing apparatuses are operating, a power loss of the radio frequency power incurred by a transmission line extending from the radio frequency generator to the electrodes in the processing chamber varies depending on each apparatus even though the output power of the radio frequency generator coupled to each processing apparatus is the same. Therefore, the real radio frequency power applied to the electrodes is not necessarily constant. The different length of the power cable forming the transmission line or the peripheral environment with different electrical characteristics of the plasma processing apparatuses contributes to a variation of the power loss.
If the real radio frequency power applied to the electrodes of the processing chamber varies depending on each plasma processing apparatus, the plasma state in the processing chamber of each apparatus varies, which may result in a variation of accuracy of the processing result of each plasma processing apparatus. Due to this, the radio frequency generator was conventionally calibrated when it is added or replaced so that the output power of the radio frequency generator was adjusted to compensate for a power loss by the transmission line.
The conventional calibration method of the radio frequency generator will be described with reference to the accompanying drawings. FIG. 8 shows a block diagram showing a configuration of a conventional radio frequency generator system for calibrating a radio frequency generator. As shown in FIG. 8, a radio frequency generator 10 is connected to a dummy load (virtual load) 30 via a coaxial cable 20 coupled to a power output terminal 12 for conventional calibration. The dummy load 30 has the same impedance as the combined impedance, e.g., 50Ω, of a real matching unit, to which the radio frequency generator 10 is connected, and the plasma processing apparatus. That is, by connecting the dummy load 30 instead of the real matching unit and plasma processing apparatus, the calibration of the radio frequency generator 10 can be performed efficiently. Further, since the impedance of the load does not change during the calibration process, the calibration result is more reliable.
On the other hand, a power meter 40 is interposed between the end portion of the coaxial cable 20 and the dummy load 30. The power meter 40 measures radio frequency power fed to the dummy load 30.
Connected to the radio frequency generator 10 is a generator control unit 50 for controlling the output power thereof. For example, the generator control unit 50 is included in a controller for controlling the plasma processing apparatus. The generator control unit 50 transmits a power-setting voltage signal 60 to the radio frequency generator 10. The power-setting voltage signal 60 is an analog signal, e.g., having a voltage ranging from 0 V to 10 V. The radio frequency generator 10 generates radio frequency power corresponding to the voltage of the power-setting voltage signal 60 through the power output terminal 12.
Further, the radio frequency generator 10 transmits a power-monitoring voltage signal 62 indicating a voltage of the radio frequency power actually sent from the power output terminal 12 (e.g., a voltage of traveling and reflection waves) to the generator control unit 50. The power-monitoring voltage signal 62 is an analog signal having a voltage ranging, e.g., from 0 V to 10 V. That is, the radio frequency generator 10 outputs the power-monitoring voltage signal 62 corresponding to the voltage of the radio frequency power sent from the power output terminal 12.
The generator control unit 50 includes a display unit (not shown) where the real radio frequency power outputted from the power output terminal 12 (output power level) and set power level according to the voltage level of the power-monitoring voltage signal 62 transmitted from the radio frequency generator 10 are displayed. In this case, the set power level is constant since a specified value is displayed. However, the radio frequency power actually outputted from the power output terminal 12 changes if the output power is changed in calibration of the output power. Due to this, the displayed set power level and the displayed radio frequency power actually outputted are not equal to each other after the output power has been calibrated. Therefore, in this case, it is necessary to adjust the displayed radio frequency power to become the set power level.
A method for calibrating the radio frequency generator 10 using such a radio frequency power system will be described hereinafter. According to this method, the output power of the radio frequency generator 10 is adjusted by the power-setting voltage signal 60 which is an analog signal so that the power actually inputted to the dummy load 30 reaches the set power level of radio frequency power.
First, if an operator enters a power-setting voltage level for the set power level of radio frequency power to an input unit (not shown) of the generator control unit 50, the generator control unit 50 transmits a power-setting voltage signal 60 indicating the power-setting voltage level set in the input unit to the radio frequency generator 10.
FIG. 9 is a graph showing a relationship between the power-setting voltage level to determine the output power of the radio frequency generator 10 and the radio frequency power actually outputted from the power output terminal 12 (output power level) before the output power is calibrated. As shown in FIG. 9, if the radio frequency generator 10 has a rated output of 3000 W, the power-setting voltage level ranging from 0 V to 10 V corresponds to the radio frequency power ranging from 0 W to 3000 W generated from the radio frequency generator 10. That is, when radio frequency power of 1700 W is generated from the radio frequency generator 10, it is noted that a power-setting voltage signal 60 of 5.67 V (≈1700÷3000×10) is fed to the radio frequency generator 10.
The radio frequency generator 10 transmits radio frequency power of 1700 W corresponding to the power-setting voltage signal 60 to the dummy load 30 via the coaxial cable 20. However, due to a power transmission loss caused by the coaxial cable 20, the radio frequency power actually supplied to the dummy load 30 is smaller than 1700 W, the radio frequency power generated from the radio frequency generator 10.
Therefore, the operator adjusts the output power level of the power output terminal 12 by changing the power-setting voltage level inputted to the input unit of the generator control unit 50 so that the radio frequency power actually supplied to the dummy load 30, i.e., the radio frequency power measured by the power meter 40, reaches the set power level. By calibrating the radio frequency generator 10 in this manner, the radio frequency power calibrated to the set power level is fed to the dummy load 30. Then, if the dummy load 30 is disconnected and the matching unit is reconnected to the coaxial cable 20, the radio frequency power calibrated to the set power level is supplied to the matching unit.
(Patent Document 1) Japanese Patent Application Publication No. 5-205898
(Patent Document 2) Japanese Patent Application Publication No. 11-149996
(Patent Document 3) Japanese Patent Application Publication No. 2003-224112
(Patent Document 4) Japanese Patent Application Publication No. 2003-032064
However, according to the conventional correction method of the radio frequency generator 10, the operator has to adjust the power-setting voltage level fed to the input unit of the generator control unit 50 until the radio frequency power supplied to the dummy load 30 reaches the set power level. Further, since an overshoot occurs if the power-setting voltage level is changed too much, the operator has to gradually change the power-setting voltage level several times while monitoring the power meter 40. Accordingly, the calibration process takes a long time.
Furthermore, as described above, since the radio frequency power actually outputted from the power output terminal 12 (output power level) is displayed on the display unit of the power control unit 50, the display of the radio frequency power changes while the output power is being adjusted. Therefore, while the output power is being adjusted, the display of the radio frequency power should be constantly changed to follow the displayed set power level. This process is troublesome in that the calibration process takes more time.
Further, since the power-setting voltage signal 60 transmitted to the radio frequency generator 10 from the generator control unit 50 is an analog signal, it is easily affected by noise. Particular when the set power level of radio frequency power becomes smaller, the power-setting voltage level for adjusting the output power becomes accordingly smaller, which aggravates the noise problem.
In case of the example shown in FIG. 9, if the power-setting voltage signal 60 is overlapped with noise of 10 mV, the radio frequency generator 10 generates the normal radio frequency power added with radio frequency power of 30 W corresponding to the power-setting voltage level of 10 mV. For example, if the normal radio frequency power is 1700 W, noise of 30 W corresponds to approximately 1.8% of 1700 W. However, if the normal radio frequency power is within a low power output range (e.g., if it is 100 W), the noise of 30 W corresponds to 30% of 100 W.
That is, if the target radio frequency power is low, the power-setting voltage signal 60 is significantly affected by the overlapped noise and therefore it may be difficult to accurately control the output power level of the radio frequency generator 10.
Conventionally, techniques for controlling a radio frequency generator during plasma processing such that radio frequency power actually applied to an electrode in a processing chamber reaches a specified value have been proposed to compensate for a power loss by a transmission line. For example, Patent Document 1 discloses a technique for performing plasma processing by supplying radio frequency power to an electrode in a processing chamber from a radio frequency generator via a cable and a matching unit and then controlling the radio frequency generator during plasma processing so that measured radio frequency power actually applied to the electrode reaches a specified value.
Further, Patent Document 2 discloses a technique for performing plasma processing by supplying radio frequency power to an electrode of a plasma reactor from a radio frequency generator via a cable and a matching unit and outputting power added with reflection wave power from the radio frequency generator during plasma processing by measuring the radio frequency power (reflection wave power) actually reflected from the electrode.
In addition, techniques for maintaining real radio frequency power applied to an electrode in a processing chamber at a specified level by controlling a matching circuit disposed between a radio frequency generator and the electrode have been proposed. For example, Patent Document 3 discloses a technique for maintaining real radio frequency power applied to an electrode at a specified level by measuring a current actually flowing to the electrode during plasma processing and by controlling a radio frequency generator or a matching circuit based on the measured current. Further, Patent Document 4 discloses an impedance matching device where power actually supplied to a load is maintained at a specified level by maintaining a circuit passing power loss of a short-circuit resistance at a set power level by controlling the power consumption of the built-in secondary inductance circuit according to power actually supplied to the load during plasma processing.
According to these techniques, the radio frequency power actually applied to the electrode is controlled during plasma processing. Herein, the techniques have been conceived to maintain a constant value of the radio frequency power at the point closest to the electrode to which radio frequency power is finally supplied (i.e., at the point near the electrode seen from the matching unit). Therefore, this is different from the above-described method for calibrating the radio frequency generator by adjusting the output power so that the power fed to the matching unit reaches the set power level when the radio frequency generator is replaced.
Furthermore, attenuation characteristics of the radio frequency power during the transmission thereof from the radio frequency generator to the electrode in the processing chamber are not uniform. For example, if radio frequency power from a radio frequency generator is transmitted to a matching unit via a coaxial cable and then supplied from the matching unit to an electrode in a processing chamber, attenuation characteristics of the radio frequency power in the matching unit and front stage are affected by many parameters unlike the coaxial cable. To be specific, attenuation characteristics for the coaxial cable are largely irrelevant to the size of the radio frequency power and therefore the radio frequency power of any value is attenuated at a constant rate. On the contrary, since attenuation characteristics of the matching unit are depending on the size of the radio frequency power, if the radio frequency power changes, it is likely that its attenuation rate is significantly changed.
Due to this, in case of using only the above-described techniques for adjusting the output power during plasma processing, it is difficult to maintain the radio frequency power applied to the electrode of the processing chamber at a specific value. Further, when the radio frequency generator is added or replaced, the radio frequency power fed to the matching unit should be accurately adjusted by completing the calibration of the radio frequency generator and, after that, the power adjustment of the matching unit and front stage should be performed. Otherwise, it is very difficult to adjust the radio frequency power actually supplied to the electrode in the processing chamber with high accuracy.
Therefore, it is more effective to use the above techniques for adjusting the output power during plasma processing after calibrating the radio frequency generator such that the radio frequency power fed to the matching unit from the radio frequency generator via the transmission line reaches the set power level.